Optical devices that affect and modify the propagation of luminous energy, such as lasers and light-emitting diodes (LED), with the purpose of homogenizing or redistributing its energy content at a location farther away from the device are generally referred to as diffusers or beam shapers. Such devices are broadly characterized by a surface pattern that contains height variations that alter the direction of propagation of the incoming illumination in typically random directions within a certain range which depends on the features of said surface pattern. A common example of diffuser commercially available is ground glass, produced by the roughening of one of its surfaces. Because of the uniformly random distribution of height variations on its rough surface, ground glass spreads the incident illumination with a Gaussian dependence against scatter angle. Holographic diffusers, formed by exposure of laser speckle, also exhibit Gaussian scatter. For example, a description of holographic diffusers can be found in K. D. Bonin and M. A. Kadar-Kallen, “Simple diffuser for production of laser speckle,” Applied Optics 28, 5293-5297 (1989). Ground glass and holographic diffusers can be thought of as belonging to the same family of devices characterized by a surface with a uniformly large distribution of random height variations. Diffusers of this type generally lead to Gaussian scatter.
Gaussian diffusers find the most use in the homogenization of light. For instance, LED sources typically produce strongly uneven illumination. A Gaussian diffuser can be used to make the LED light more homogeneous. However, homogeneity does not necessarily imply a high degree of uniformity or efficiency. A Gaussian profile can only provide high uniformity of light distribution with very low efficiency and, conversely, high efficiency with very non-uniform light distribution. Uniform light in the present context means light distribution with small variation of the measured intensity and efficiency means the fraction of the incident illumination that is concentrated over a specified angular range or region of space.
Another class of diffusers, diffractive diffusers, are based on the principles of interference and diffraction and can be designed to produce fairly general scatter patterns. A simple discussion of diffractive diffusers can be found in D. R. Brown, Beam shaping with diffractive diffusers in Laser Beam Shaping, F. M. Dickey and S. C. Holswade, eds., Marcel Dekker, New York, 2005, pp. 249-271, Chap. 6. Depending on the design approach and the symmetry properties of the pattern to be produced high efficiency is possible, 80-90%, and in some cases even higher. However, because diffractive elements achieve light spread through feature sizes, they are limited to relatively small diffuser angles. As the desired angular spread increases the required feature size for the diffractive element decreases and it becomes more difficult to produce the element. In addition to this practical difficulty, diffractive elements are generally limited to single wavelength operation, and if the spectrum of the incident illumination includes several spectral lines the zero order of the diffraction pattern produced by the diffractive diffuser increases beyond the maximum value of all the other diffraction orders, sometimes several times the maximum of the other diffraction orders, as a result negatively effects light diffusing performance. This phenomenon is generally referred to as the “zero-order problem.” Although in some cases it is possible to design a diffractive element that operates over a broad continuous spectral band, such as disclosed in U.S. Pat. No. 6,118,559, fabrication difficulties persist as well as the need for coherent illumination. As a result, diffractive diffusers are typically limited to specialized applications diffusing monolight coherent illumination.
There is thus a necessity for a diffuser that can be employed in general applications and enable uniform scatter patterns for monochromatic or broadband illumination with high efficiency. There are several examples of diffuser components that attempt to address this issue. The mixing rod provides a simple example where light is coupled in one end of the rod and comes out of the opposite end, after propagating through the rod by means of total internal reflection. Unfortunately, mixing rods can only provide good uniformity for specific patterns, specifically, simple patterns that completely fill the plane. For example, mixing rods can produce square and hexagonal patterns with good uniformity but cannot generate a uniform round pattern. Furthermore, mixing rods are in a sense volume diffusers that require a minimum volume extent to ensure a certain degree of uniformity. As a result, these are not very compact devices. Also, periodic microlens arrays have also been used to some degree as diffusers. Either single microlens arrays or fly's eye arrangements have been reported for producing uniform scatter patterns with high efficiency (see, for example, “Wave optical analysis of light-emitting diode beam shaping using microlens arrays,” Opt. Eng. 41 (2002) 2393-2401). However, similar to mixing rods, the periodic nature of the array implies that it can only be used to produce patterns that can perfectly cover the plane such as square, rectangular, or hexagonal. A periodic microlens array cannot, for instance, produce a uniform circular pattern. Furthermore, since a periodic microlens array is a grating, it gives rise to diffraction artifacts that can be objectionable in some applications and may require the use another weak diffuser to work in connection with the microlens arrays with the purpose of randomizing the observed diffraction pattern.
Another approach, such as that described in “Photofabrication of random achromatic optical diffusers for uniform illumination”, Appl. Opt. 40, 1098-1108 (2001), describes a diffuser where the surface pattern is formed by piecewise linear segments. Although effective in principle, such approach does not seem practical as it requires large illumination areas to produce a surface that covers the continuum of slopes required for uniform illumination. Random microlens arrays, such as discussed in U.S. Pat. Nos. 7,033,736 and 6,859,326, can address these issues and be used for general beam shaping, including producing uniform scatter patterns. The design process, however, can be complex since it requires a random distribution of microlens units with specified probability density functions that need to be optimized to achieve the desired scatter properties.
Together with high uniformity, it is generally desirable that the diffuser device exhibit high efficiency. That is, most of the light should be directed towards the desired target angular range or region where high uniformity is needed. There are three basic regions associated with a general scatter pattern. FIG. 1 illustrates these concepts using the cross-section of a scatter pattern from a random microlens diffuser. The target angular region represents the domain over which high efficiency and uniformity is required. In the example of FIG. 1 the target region is in the angular range of ±7.2 degrees. The fall-off region represents the transitional domain where the intensity falls from the levels at the target region down to zero. Since the intensity rarely falls to zero it is common to utilize a definition such as the extent of the region where the intensity falls from 90% to 10% of the average value within the target region. In the case of surface diffusers the character of the fall-off region is determined by the point spread function of the elementary scatter elements that compose the diffuser. The domain beyond the fall-off region is the region of scatter loss or high diffraction orders and generally contains light lost due to fabrication imperfections, roughness, or high diffraction orders in the case of diffractive elements. The amount of energy in this last region can be in principle minimized in the case of refractive elements, such as microlens-based diffusers, by fabrication improvements. In the case of diffractive elements this is not necessarily true since high-order energy loss is often a natural consequence of the design process itself.
As mentioned previously, Gaussian diffusers such as ground glass and holographic can be problematic when there is a need for both high uniformity and efficiency. Diffractive diffusers produce diffraction orders that generally contain 80-90% of the incoming light within the target region lost to higher orders. The remainder falls outside of the target region. The point spread function may be determined in the particular case of binary diffractive diffusers where the surface is composed of only two constant height levels. As is well-known from the theory of diffraction (see, for instance, J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill, New York, 1996) an aperture that imparts a constant phase delay produces the so-called diffraction-limited spot in the far-field or at the focus of a lens. In the case of a round aperture this is called an Airy disk pattern. The diffraction-limited point spread function is narrowest for a square or simple slit where the angular width of the diffraction pattern is ±λ/D, where λ is the wavelength of the incident illumination and D is the beam size. At the edge of the scatter pattern, the intensity fall-off corresponds to the intensity fall-off of the point spread function of the elementary scatter elements. Therefore, given that the diffraction-limited point spread function is very sharp, for a diffractive element the major source of efficiency loss is due to higher-order light directed outside of the target.
A microlens-based diffuser, on the other hand, does not intrinsically scatter light outside of the target region. In practice, fabrication limitations lead to wide-angle scatter outside of the target region but this source of loss may be minimized by improving manufacturing techniques. However, the point spread function of a microlens unit is considerably wider than the diffraction limit and, as a result, the efficiency of a microlens diffuser is mostly limited by the energy in the fall-off region. The case of a microlens diffuser is thus opposite to that of a diffractive diffuser with the absence of intrinsic loss to higher diffraction orders but wider intensity fall-off because of the point spread function of the elementary lens unit.
Thus, it would be desirable to provide light diffusing optical devices that enable substantially uniform scatter patterns for monochromatic or broadband illumination along a target angular range or region that is efficient in minimizing light falling outside of the target angular range, and moreover is useful for diffusing both coherent and non-coherent illumination, and do rely on use of gratings associated with diffractive diffusers.